THE MANTLE

                           THE MANTLE

It is separated from the over - lying crust by the Mohorovicic - Discontinuity which is a first order discontinuity. Its thickness is about 2865 kms. It forms 83% of the earth by volume and 68% by mass. 

It is the source - region of most of earth's internal energy and of forces responsible for ocean - floor spreading, continental drift, orogeny and major earthquakes. 

The material is olivine - pyroxene complex, which exists in solid state. It is believed that the upper mantle has a mix of 3 parts of ultramafic rocks and 1-part of basalt and the mix is known as Pyrolite. The lower mantle extends from 1000km to the core boundary.

Within the mantle, a number of second - order discontinuities have been located, which are as follows : 

1) Density break at 80 km depth : Density changes from 3.36 to 3.87 above and below the level respectively. 

2) Gravity break at 150 km depth : gravity changes from 984cm/ sec² till it reaches at a depth of 1200 kms. 

3) At 700 km - depth, there changes the capability of the materials in storing amount of elastic - stain energy. Up to 700 kms the capability is more.

4) Repetiti discontinuity : At 950 km depth. It marks the lower limit of the very rapid rise in the velocity of seismic vibrations.

5) Gravity - break : At 1200 km depth, gravity attains its minimum value i.e., 974cm/sec² , therefore it rises upto 1068 cm/ sec² at the core boundary.